address review (claude): trim prior-issue prose tail from the head so the closing survives

trimContextToBudget keeps the head of the string, so on a small-context
local model the prior-issue prose tail lost its literal closing lines —
the exact seam the prose template tells the model to open from. Size the
tail via extractProseTail (which keeps the end) capped at the budget
instead, and pin it with a regression assertion that the trimmed tail
contains the closing marker, not the opening.

Author: atomantic

server/services/pipeline/textStages.js

@@ -274,9 +274,9 @@ async function buildProseContextAugment(series, issue, options = {}) {
   const empty = { priorIssueProseTail: '', nextIssueBeats: '', hasNeighborContinuity: false };
 
   const { prior, next } = await resolveVolumeNeighbors(series, issue);
-  const rawTail = prior ? extractProseTail(stageContentOf(prior.stages?.prose)) : '';
+  const priorProse = prior ? stageContentOf(prior.stages?.prose) : '';
   const rawBeats = extractNextIssueBeats(next);
-  if (!rawTail && !rawBeats) return empty;
+  if (!priorProse && !rawBeats) return empty;
 
   // Token-budget both blocks so they degrade by trimming on a small window
   // rather than blowing the prompt. Resolve the prose stage's planning window
@@ -294,8 +294,15 @@ async function buildProseContextAugment(series, issue, options = {}) {
   const continuityChars = Math.max(0, Math.floor(usableTokens * CONTINUITY_BUDGET_FRACTION)) * CHARS_PER_TOKEN;
 
   // Prose tail keeps priority; next beats reclaim whatever the tail doesn't use.
+  // The tail must be trimmed from its HEAD (keep the actual CLOSING of the prior
+  // issue — the seam the template tells the model to flow from), so size it via
+  // extractProseTail (which keeps the end) rather than trimContextToBudget (which
+  // keeps the head and would discard the literal final lines on a small window).
+  // Never grow past PRIOR_PROSE_TAIL_CHARS just because the window is large.
   const tailBudget = rawBeats ? Math.floor(continuityChars * PRIOR_TAIL_SHARE) : continuityChars;
-  const priorIssueProseTail = rawTail ? trimContextToBudget(rawTail, tailBudget) : '';
+  const priorIssueProseTail = priorProse
+    ? extractProseTail(priorProse, Math.min(PRIOR_PROSE_TAIL_CHARS, tailBudget))
+    : '';
   const beatsBudget = continuityChars - priorIssueProseTail.length;
   const nextIssueBeats = rawBeats ? trimContextToBudget(rawBeats, Math.max(0, beatsBudget)) : '';
 

server/services/pipeline/textStages.test.js

@@ -634,7 +634,9 @@ describe('pipeline text stage generator', () => {
     });
     const { series } = await seed();
     const sea = await seasonsSvc.createSeason(series.id, { title: 'V1' });
-    const bigTail = 'The tide rolls in. '.repeat(2000); // ~38K chars — far over the window
+    // Distinct opening vs closing markers so we can prove the trim keeps the END
+    // (the actual close the model must flow from), not the head.
+    const bigTail = `PRIOR_OPENING. ${'The tide rolls in. '.repeat(2000)}PRIOR_CLOSING.`;
     await issuesSvc.createIssue({
       seriesId: series.id, title: 'A', seasonId: sea.id, arcPosition: 1,
       stages: { prose: { status: 'ready', output: bigTail } },
@@ -646,6 +648,10 @@ describe('pipeline text stage generator', () => {
     // Present but trimmed below the raw ~2000-char tail cap — degraded, not dropped or errored.
     expect(ctx.priorIssueProseTail.length).toBeGreaterThan(0);
     expect(ctx.priorIssueProseTail.length).toBeLessThan(2_000);
+    // Regression guard: the trim must keep the CLOSING of the prior issue (the
+    // seam the prose template tells the model to open from), not its opening.
+    expect(ctx.priorIssueProseTail).toContain('PRIOR_CLOSING');
+    expect(ctx.priorIssueProseTail).not.toContain('PRIOR_OPENING');
   });
 
   it('prompt context carries derived lengthTargets for the custom profile', async () => {